The 30A 120V RV-use plug is 3 prongs, not 4. I agree, that looks like a 50A plug in the picture. *IF* it's wired correctly, it could supply your trailer if plugged into a 50A outlet, but I would want to determine that it's wired correctly or that the trailer wiring had been upgraded to properly handle 50A service before I plugged it in.
Here is a useful site
with lots of info about RV electrical systems. Here is a link directly
to the 30A page which has pictures of 30A plugs and receptacles.